It is often desirable to exercise the skeletal muscles responsible for supporting and moving various parts of the body in order to strengthen such muscles. Exercise may be undertaken for rehabilitative purposes to restore normal strength and range of motion where muscle tissue has been weakened by disease, injury or prolonged inactivity as commonly occurs when a body part has been immobilized for an extended period in a cast or brace. In otherwise healthy individuals, exercise is beneficial for increasing vigor and maintaining an attractive appearance. For athletes or others engaging in demanding physical activity, exercise improves performance and is believed to reduce the likelihood of injury.
A skeletal muscle can exert a force effective to move a part of the body only by contracting and shortening in length. Once contracted, force must be applied in order to lengthen the muscle and restore the body part to its initial position. Hence, skeletal muscles are usually arranged around a joint in antagonistic pairs, so that when one muscle contracts, another is lengthened moving the body part in a plane of rotation about a skeletal joint. Most parts of the body, particularly the limbs, are supported and moved by more than a single antagonistic pair of skeletal muscles and are capable of motion in more than one plane. As a consequence, most external body movements can be described in terms of rotational movement about one or more axes of one or more skeletal joints. In the foot for instance, twelve muscles serve to support and move the foot about the ankle. Movement in different directions involves different muscles, some of which are naturally capable of exerting more force than others. This is due to a variety of factors including difference among muscle length and bulk and, significantly, differences in overall mechanical advantage related to the position of the muscle. For example, the muscles responsible for plantarflexion and dorsiflexion of the foot, i.e., moving the foot up and down, are stronger than the muscles which perform inversion and eversion, i.e., rotation of the foot to the inside and outside, respectively. A major shortcoming of exercise machines of the prior art is that they do not adequately accommodate differences in the forces with which a part of the body can be rotated about different axes.
For example, U.S. Pat. No. 4,186,920 to Fiore et al. shows an exerciser wherein the foot is strapped to a foot support connected to a ball and socket universal joint to permit tilting in any direction. The ball and socket joint provides adjustable frictional resistance by virtue of a two-piece socket which can be tightened against the ball to provide a desired amount of resistance. While such an apparatus allows universal rotational movement of the foot, the resistance in every direction is substantially the same. If the resistance is set sufficiently high to resist the more powerful plantarflexion/dorsiflexion movements, it will be undesirably high for inversion/eversion movements. Conversely, if the resistance is lowered to a proper level for inversion/eversion, the exercise will be less effective for plantarflexion/dorsiflexion. This dilemma is most acute when the physical therapist or athletic trainer wishes to combine plantarflexion/dorsiflexion and inversion/eversion movement components into a single motion wherein the toes traverse an elliptical or circular path. If the resistance is set correctly for one component, it will be incorrect for the other.
Another problem with the exerciser shown in Fiore et al. is that at high resistance settings, the apparatus may tip or slide if the weight of the base is insufficient to firmly anchor the device to the floor.
U.S. Pat. No. 4,605,220 to Troxel seeks to isolate the muscles involved in plantarflexion, dorsiflexion inversion and eversion by providing four independently adjustable shock absorbers disposed to resist rotation about a pair of mutually perpendicular horizontal axes. While such an arrangement can be used to provide more resistance for dorsiflexion/plantarflexion than inversion/eversion movements which are normally less powerful, it has certain shortcomings. Shock absorbers typically operate using a compression spring or dashpot or some combination of the two. Springs as resistance generators are undesirable since the force will vary with the position of the foot as displacement of the spring changes. The degree of resistance provided by dashpot devices is speed dependent. Thus, dashpots are unsuitable where it is desired to provide a substantially constant resistance over a range of speeds.
Various types of friction devices for supplying resistance for exercising are known (see, e.g., U.S. Pat. No. 3,717,338 to Hughes). However, a problem faced by some exercise machines utilizing friction resistance means is the difficulty in obtaining smooth resistance over a wide range of resistance settings without requiring an unduly large frictional contact area. For example in the friction clutch illustrated in U.S. Pat. No. 3,103,357 to Berne, two stationary and one rotating disk are required to provide sufficient frictional contact area to supply the torque required to adequately resist bending of the knee. The smaller the frictional contact area, the greater the pressure needed between frictionally mating surfaces to produce the same resistance. Unless the properties of the frictional surfaces are carefully controlled, high pressures can cause static frictional effects to become highly apparent and sometimes dominate dynamic frictional effects. This can result in the exercise machines having an uneven or "jerky" feel, particularly during slow movements or at the beginning of motion where static frictional forces must be overcome.
Accordingly, there is a need for an exercise machine which accounts for differences in the force with which a part of the body can be rotated about different axes. Furthermore, there exists a need for such an exercise machine which provides resistance which does not vary with position or velocity. There is also a need for an exercise machine having frictional resistance means which can provide adjustable, smooth resistance over a wide range of resistance settings without requiring unduly large frictional surfaces. In addition to meeting the above needs, it is an object of the invention to provide an exercise machine having a rotatable friction plate and a non-rotatable plate carried by a common rotating shaft so that the plates are urged together by a compression spring whose length can be adjusted by rotating a jam nut threaded on the shaft and wherein the jam nut is prevented from unintentionally unthreading as the machine is used. It is a further object of the invention to provide an apparatus for exercising the muscles associated with movement of the foot wherein at least one axis of rotation of the foot can be adjusted to align with the natural angulation of the foot and which is not subject to tipping or sliding when used even at high resistance settings. These and other objects and advantages of the present invention will be more fully appreciated as the reader proceeds.